power analysis......

[Fulthrotl]

i've got three different facilities with power issues,
or suspected power issues.... so i'm looking at
renting a fluke 435 for a month......

anyone have suggestions on something better?

got a "drive by and look at it" call yesterday, from
two different customers with three different facilities.
odd enough i was wondering if it's a full moon..

same complaint in each. "loud transformer humming."

first one i got to, no kidding..... 50' away, down a hallway
from the electrical room, you heard this weird humming...
went inside the electric room, 300 kva dry type xfmr, sounding
like it wanted to grenade on the spot.... transformer is spec'd
at 55 db at full load, this one is measuring 82 db, 6' away from
front of unit.

but the iphone app i was using, is a spectrum analyzer, and
considering what it is, fairly accurate... and it showed this
weird low frequency oscillation about twice a second... the
125 hz line was double the rest of the spread, but the frequencies
below that were surging up to 80 db and falling off to 40 db.

sounded like the soundtrack for a bad science fiction movie,
right before some evil spacecraft lands and horrible things
happen... :-/

anyway, anyone have equipment and troubleshooting suggestions?
i'm gonna do a PM on the xfmr's with a megger, to at least make
sure it's not about to grenade, but suggestions would be appreciated.

thanks

 

[Besoeker]

i've got three different facilities with power issues,
or suspected power issues.... so i'm looking at
renting a fluke 435 for a month......

anyone have suggestions on something better?

got a "drive by and look at it" call yesterday, from
two different customers with three different facilities.
odd enough i was wondering if it's a full moon..

same complaint in each. "loud transformer humming."

first one i got to, no kidding..... 50' away, down a hallway
from the electrical room, you heard this weird humming...
went inside the electric room, 300 kva dry type xfmr, sounding
like it wanted to grenade on the spot.... transformer is spec'd
at 55 db at full load, this one is measuring 82 db, 6' away from
front of unit.

but the iphone app i was using, is a spectrum analyzer, and
considering what it is, fairly accurate... and it showed this
weird low frequency oscillation about twice a second... the
125 hz line was double the rest of the spread, but the frequencies
below that were surging up to 80 db and falling off to 40 db.

sounded like the soundtrack for a bad science fiction movie,
right before some evil spacecraft lands and horrible things
happen... :-/

anyway, anyone have equipment and troubleshooting suggestions?
i'm gonna do a PM on the xfmr's with a megger, to at least make
sure it's not about to grenade, but suggestions would be appreciated.

thanks

I guess the 125Hz might be close enough to 120Hz (depending on measurement accuracy) to be twice supply frequency so maybe no surprise there.
But 82dB is a helluva racket. My experience with noisy transformers has mostly been a result of loose laminations.
Maybe the clamps have become slack due to vibration.

 

[jim dungar]

By far and away the most common source for noise in air cooled transformers is the steel core, it is rarely the windings themselves. When you see hoof prints, think horses not zebras.

The two most common sources are poor clamping of the laminations or vibration transmission to the mounting surface.
The lamination clamping bolts may be loose, or rust may have formed between the individual laminations.
The core and coil assembly should be not be solidly fastened to the structure, it needs to float. The original noise isolation pads may have failed.

And of course the load profile (i.e. pulsing loads like welding) can be a factor also.

 

[Fulthrotl]

I guess the 125Hz might be close enough to 120Hz (depending on measurement accuracy) to be twice supply frequency so maybe no surprise there.
But 82dB is a helluva racket. My experience with noisy transformers has mostly been a result of loose laminations.
Maybe the clamps have become slack due to vibration.

yeah... here's what it sounds like..... dunno how the sound level will come out
on peoples computers, but this was showing 82 db.

 

[Fulthrotl]

By far and away the most common source for noise in air cooled transformers is the steel core, it is rarely the windings themselves. When you see hoof prints, think horses not zebras.

The two most common sources are poor clamping of the laminations or vibration transmission to the mounting surface.
The lamination clamping bolts may be loose, or rust may have formed between the individual laminations.
The core and coil assembly should be not be solidly fastened to the structure, it needs to float. The original noise isolation pads may have failed.

And of course the load profile (i.e. pulsing loads like welding) can be a factor also.

yeah, if the laminations, the E's and I's, aren't really flat, they will flutter, creating a 60 hz hum.
it'll get worse with time, as the whole thing gets dipped during manufacture, and the laquer,
or shellac, or whatever product the manufacturer chose to dip the core with breaks down.

when they make the laminations, they are supposed to be flat within .010". on large stampings,
that is really hard to maintain, as they have to unreel and detension the stock before stamping.

(last job i had before becoming an apprentice was at a company that did, of all things, stamping
transformer laminations, in QC.)

if you got a core with super flat laminations, the transformer is almost silent. you can barely hear it.
but this thing sounds like a fork in a garbage disposal.

 

[Besoeker]

yeah... here's what it sounds like..... dunno how the sound level will come out
on peoples computers, but this was showing 82 db.

It's humming 'cos it knows the tune but not the words?

Slightly more seriously, you can hear the hum but it probably doesn't do it justice compared to hearing it in real life.

 

[Fulthrotl]

It's humming 'cos it knows the tune but not the words?

Slightly more seriously, you can hear the hum but it probably doesn't do it justice compared to hearing it in real life.

yeah, the low frequency thrumming sound is what carries.... you
can hear it clearly in the elevator lobby at the front of the building.

most core's i have seen recently have been riveted, not bolted up,
so i don't know what if anything can be done about this if it's riveted.
maybe the core is bolted and can be torqued, and all will be better.

i quoted on a PM, so i am done until i get a purchase order in my inbox.

 

[T.M.Haja Sahib]

Ferroresonance?

Ferroresonance?

One more possibility for noise in transformer is ferroresonance.Needs to checked up for this too........

 

[Speedskater]

On the hi-fi forums, I often read complaints about their equipment having transformer humming caused by DC on the AC power line. Really it might be harmonic distortion on the power line causing asymmetry or a DC offset. Or it may be a higher line voltage than the transformer was designed for. The following paper is from a company that makes very nice (expensive) transformers for many purposes.

http://www.idc-online.com/technical...MEASURING_ACOUSTIC_NOISE_EMITTED_BY_POWER.pdf

Or sometimes these hi-fi discussions use the primitive example of DC offset caused by a hair-dryer switching to low power by inserting a diode is series with the heating element and half-wave rectifying the current. I have read (I think that it was in "Power Electronics Technology" magazine) about industrial welders having diode failures causing similar problems.

 

[jim dungar]

On the hi-fi forums,...

I have very little respect for audiophiles discussing AC power systems.
Transformer hum because of magnetostriction.

 

[Speedskater]

You should see the crazy circuits the hi-fi folk make to block that DC. But the above linked transformer company paper makes high-end industrial and hospital transformers.

http://www.plitron.com/

 

[Speedskater]

Some of those DC blocker circuits:

http://www.diyaudio.com/forums/powe...t-buzzing-toroid-transformers-what-right.html

 

[Pullnwire]

I have a customer with the same trans doing the same thing, except at random intervals, it sounds like there is a pile of 1/4" washers vibrating around on the lid. THey wont even let me pull the cover off it. They said they will deal with it when it fails...... Soo, I can drive to the supply house and just pull a 300KVA trans right off the shelf and toss into my pickup, right???

 

[Fulthrotl]

I have a customer with the same trans doing the same thing, except at random intervals, it sounds like there is a pile of 1/4" washers vibrating around on the lid. THey wont even let me pull the cover off it. They said they will deal with it when it fails...... Soo, I can drive to the supply house and just pull a 300KVA trans right off the shelf and toss into my pickup, right???

i keep a spare one on the truck.... don't you?

edit:
you are in orange crate as well.... if yours goes first,
give me a call and you can borrow my spare... no point
in both of us carrying one around, is there?

 

[Pullnwire]

Good Idea Randy! I know if I even hint to my supplier I might need one asap, and have one transferred in, this thing will last for 20 years. Until then, I'll count on your spare......

Did I mention mine has copper windings? I might dismantle it in place, for ease of removal of course......

 

[Fulthrotl]

Good Idea Randy! I know if I even hint to my supplier I might need one asap, and have one transferred in, this thing will last for 20 years. Until then, I'll count on your spare......

Did I mention mine has copper windings? I might dismantle it in place, for ease of removal of course......

if you need some help with carrying that scrap metal off, let me know... i know that can be a chore.....

copper core? geez... the price i got on an aluminum core was almost $8k out the door..... i don't even
want to think about copper...